1. Field of the Invention
The present invention relates to a transversal type automatic equalizer system with tap coefficient protection. The system according to the invention is used for automatic equalization as one of various compensation techniques, for example, for digital signal communication in voice band frequency. The system according to the invention can be used not only for a fractionally tap spaced equalizer (FSE), but also for a usual transversal type automatic equalizer.
2. Description of the Related Arts
A system of a fractionally tap spaced equalizer (FSE) as one of the transversal type automatic equalizer systems has been known in which the sampling rate T' for defining the tap interval is set to be less than the symbol rate T defined by the reciprocal of twice the highest signal frequency. The fractionally tap spaced equalizer having the tap interval T' equal to half the symbol rate T (T'=T/2) is known as a double sampling equalizer.
In the case where the FSE is used in the receiving portion of a voice frequency range modem for the frequency range of about 0.3 to 3.4 KHz, the speed of the pull-in operation of the automatic equalizer in the receiving portion is high, since the operation does not depend on the sampling phase of the processing in the receiving portion.
In the FSE, however, it happens that the tap coefficients C.sub.-N to C.sub.N grow abnormally due to the indefinite range of the FSE, and the gain of the equalizer reaches the upper limit and accordingly an overflow is caused. This situation was reported, for example, in CH2114-7/85/0000-1667$1.00 C 1985, IEEE.
In the characteristic of the distribution of the spectrum of the FSE with respect to an ideal transmission pulse, the FSE has the range extended to .pi./T' and an indefinite range where the symbol power is zero is formed between the extended FSE range and the symbol range.
In order to prevent the abnormal growth of the tap coefficient caused by the indefinite range of the FSE where the symbol power is zero, an attempt has been made, for example, to force an addition of a weak noise component which does not affect the signal-to-noise ratio to the indefinite range so that the power level in the indefinite range is not made to become zero. However, in this attempt, there is a problem that the signal-to-noise ratio of the signal in the system is deteriorated.